The invention relates to an optical security element having a substrate layer in which a relief structure defined by relief parameters, in particular relief shape, relief depth, spatial frequency and azimuth angle, is shaped out in a surface region defined by an X-axis and a Y-axis, for producing an optically perceptible effect, and a system for visualising items of concealed information with such an optical security element.
The ever improving photocopying technology and the ongoing development of electronic scanning and printing apparatuses mean that there is an increasing need for optical security elements which are as forgery-proof as possible.
Now, U.S. Pat. No. 6,351,537 B1 describes an optical security element which combines a hologram and a concealed image to increase the level of copying security. The hologram used is a daylight hologram which is generated by an optical-diffraction structure shaped in a photopolymer film and is visible without the use of a monochromatic, coherent light source. The concealed image and the hologram are preferably arranged in adjacent relationship on a substrate. The concealed image is rendered visible by means of a decoding device. In that respect the decoding device used can be digital copiers or scanners but also transparent carriers on which a line grating with a line spacing corresponding to the desired scanning frequency is printed. In that case the concealed image is produced from a starting image by a procedure whereby firstly the frequency components of the starting image, which are greater than half the scanning frequency of the decoding device, are removed and the remaining frequency components are then mirrored at the frequency axis which corresponds to half the scanning frequency.
In that way the optical security element affords a first security feature, namely the hologram, and a second security feature, namely the concealed image. That enhances the level of safeguard against forgery.
U.S. Pat. No. 5,999,280 describes a holographic process for enhancing the level of safeguard against forgery, in which a concealed image which can be perceived only by means of a special decoding device is shaped in a hologram. When the decoding device is moved over the hologram the concealed pattern can then be visually detected by the viewer.
In that case such a hologram is generated in an encoding process from a background image and from the image to be concealed in the hologram. The background image comprises a line grating with a plurality of parallel black stripes. Now, in the encoding process, those parts of the image to be concealed, which are over the black stripes of the background image, are converted into white. Those parts of the image to be concealed, which are over the white part of the background image, are left black. Conversion into a hologram is effected by means of classic holographic technologies, in respect of which there are limitations in respect of the grating structures which can be produced, by virtue of the underlying physical principle.
Here however there is the disadvantage that such a security element can be imitated by the use of holographic procedures.